The precise clamping load of a threaded fastener is extremely important in determining whether or not an assembly, including the fastener, will fail in service. Consequently, threaded fasteners should be installed in a controlled manner, whereby the clamping load required to maintain the integrity of the assembly is achieved.
One common technique for controlling the tightening of threaded fasteners is to use torque control apparatus by which a specific maximum torque is applied in an attempt to attain a desired preload for particular thread and frictional conditions. Such an approach has the disadvantage that there may be variations in the torque/tension relationship from one tightening cycle to the next for the same assembly or same type of assembly due to different friction conditions, whereby varying clamping loads may be produced for a given applied torque.
Another known technique which is not dependent upon frictional conditions involves measuring the elongation of the fastener as the assembly is tightened. While this approach is capable of developing the accuracy required to achieve the desired clamping load, as a practical matter, in most cases direct measurement of elongation is either impossible or commercially unfeasible.
Yet another tightening technique which has been employed in the past in installing threaded fasteners is based on angle control. Given an estimate of the elongation required to achieve a desired clamping load, the threaded fastener is turned through a precise angle of tightening which will produce the necessary elongation. The difficulty of this approach is determining where to start measuring the angle of tightening, since it is often difficult to identify the initiation of the elongation of the fastener.
U.S. Pat. No. 3,982,419 is directed to an apparatus and method which involve tightening threaded fasteners into the yield region of the fasteners. Under such conditions, the disadvantages of the other techniques described above are avoided and the integrity of the assembly is greatly enhanced. There are, however, applications where the threaded fastener preferably is tightened to some point within its elastic range. For example, in the installation of certain high strength bolts, tightening to some clamping load below the elastic limit of the fastener will provide the desired condition.
U.S. Pat. No. 4,104,780 to Sigmund and U.S. Pat. No. 4,104,778 to Vliet are each directed to a method and apparatus for tightening assemblies held together by threaded fasteners in which the tightening technique is based upon imparting a prescribed rotation to the threaded fasteners after the starting point for the prescribed rotation has been determined by detecting that the rate of change of torque applied to the fastener with respect to rotation is a constant. Although the underlying concept in each of these two patents, in theory, will produce a joint tightened to the desired degree, certain additions and modifications need to be made to the embodiments described to render the method and apparatus in each of these patents suitable for practical implementation.